Traces of benzene in lean natural gas, having no or very small amounts of liquefied petroleum gas (LPG) and/or heavier components in the lean gas, can freeze in the gas liquefaction units operating at sub cryogenic temperatures, if the benzene is not removed from the feed to the gas liquefaction to less than 1 part per million (ppm) level. Natural gas in general and liquefied natural gas (LNG) in particular, is usually comprised mostly of methane (C1). Natural gas may also, however, contain lesser amounts of heavier hydrocarbons such as ethane (C2), propane (C3), butanes (C4) and the like, which are collectively known as C2+, or ethane plus. Hydrocarbons heavier than ethane are collectively known as C3+, or C3 plus, or propane plus. Removal of the small amounts of benzene from a very lean methane rich natural gas using a cryogenic process such as Gas Sub-cooled Process (GSP) operating at high pressure or other similar cryogenic expander based process is also difficult due to the absence of adequate amount of heavier LPG components (C3, iso-C4, n-C4, pentanes, etc.) in the feed gas to the gas liquefaction process. Small amounts of benzene are difficult to condense and remove from the lean feed gas having high methane content (over 97% methane, especially in the absence of adequate amounts of heavier (C3 plus) components).
The process for removing benzene to a very low level (less than 1 ppm) in absence of adequate amount of C3 plus components in the natural gas liquid (NGL) is described herein. Conventional cryogenic expander processes can normally remove NGL components as well as heavier components, including benzene, to a level of less than 1 ppm benzene to avoid freeze up in the natural gas liquefaction units only when sufficient heavier hydrocarbons are present in the gas mixture. These conventional cryogenic expander processes cannot economically and/or efficiently remove benzene in the absence of adequate LPG (C3, and/or C4 plus) components because the traces of benzene cannot be condensed at high operating pressure and thereby cannot be easily removed. In these instances, the gas from the overhead of the demethanizer (DeC1) going to the liquefaction will carry over benzene to a higher than 1 ppm level to the gas liquefaction section, where the lack of solubility of benzene in the LNG (methane) will cause freezing in the sub-cryogenic sections of the liquefaction process.
The inventive process and apparatus solve the problem described above through modification to a related design and operation at relatively higher pressure to remove the benzene to less than 1 ppm from lean natural gas containing very small amount of benzene in the absence of adequate LPG (C3, and/or C4 plus) components. The embodiments of the present invention described herein thereby provide economic and energy efficient methods and apparatuses to remove benzene from gaseous feeds to the liquefaction unit lacking adequate amounts of C3, and/or C4 plus components.